The present invention relates to an image formation apparatus for forming an image on a recording medium by heating and pressing an unfixed toner image on the recording medium such as recording paper or an OHP film, and particularly, to an image recording apparatus that realizes the stable feeding of a recording medium, whose feeding is difficult, such as thin paper, and thereby can deal with various type of recording media, while preventing the degradation of image quality such as offset and extending the life of a fixing device.
In a conventional image formation apparatus which forms an image with toner particles, as conventional technology concerning a fixing device that can prevent offset by an electric action and deal with various types of recording media, as described in, for instance, JP-A-11-84932, there is a fixing device that has a bias power source for the core metal or core bar of a fixing roller, and applies a voltage from this external power source to the core bar while switching the voltage between the same polarity as a recording medium and the reverse polarity according the timing of feeding of the recording medium.
The above conventional art controls the polarity of the voltage applied to the fixing roller core bar from the bias power source according to the feed timing of a recording medium, in order to prevent offset and feed various types of recording media. With this arrangement, wrapping of the leading end of a recording medium around the fixing roller is prevented by applying the voltage of the same polarity as that of the recording medium when the leading end of the recording medium passes a fixing nip, and the occurrence of offset is prevented by applying the voltage of the reverse polarity to the recording medium, that is, the voltage of the same polarity as that of toner particles, when toner is being fixed.
The arrangement of the above conventional art, however, requires the bias power source for applying the voltage to the fixing roller core bar in order to prevent the occurrence of offset. Further, it is necessary to perform such a control of applying the voltage to the fixing roller core bar and changing the polarity of the voltage according to the feed timing of a recording medium.
An object of the invention is to prevent the offset at the time of fixing an unfixed toner image which is held on a recording medium by an electric force, and to make it possible to perform the fixing that copes with various types of recording media.
The above object is attainable by each or a combination of the following items (1) to (7).
(1) A pair of fixing rollers each comprise single silicone rubber layers, which have a volume resistance value of 1013 xcexa9xc2x7cm or more and which are coated around the outer peripheries of their core bars. The surface of one of the pair of fixing rollers, which contacts with an unfixed toner image, is charged with electricity of the same polarity as that of toner particles due to frictional electrification caused during feeding of a recording medium. This construction provides an electric repulsive force for preventing the occurrence of offset.
(2) In the above-described item (1), the pair of fixing rollers forms a nip where the fixing roller contacting with the unfixed toner image has a concave shape. This causes a recording medium to be discharged along the fixing roller on which on image is formed, to enable the stable feeding of the recording medium.
(3) The pair of fixing rollers are provided with the single silicone rubber layers, whose volume resistance value is not less than 1013 xcexa9xc2x7cm, coated around the respective peripheries of their core bars, so that they are charged with electricity of the reverse polarity to that of the recording medium due to frictional electrification caused at the time of the feeding of the recording medium.
(4) The pair of fixing rollers are polarized reverse to the recording medium due to frictional electrification caused during the feeding of the recording medium. An absolute value of the quantity of charged electricity is made smaller at the roller contacting with the unfixed toner image than at the roller on the other side. Such magnitude correlation of the charged electricity quantity ensures that the recording medium is discharged along the roller on the non-image formation side, and the feeding of the recording medium becomes stable.
(5) The pair of fixing rollers in the items (3) and (4) form a nip, and at this nip, the fixing roller contacting with the unfixed toner image is in a concave shape.
(6) In the arrangement according to any of the items (1) to (5), a separation pawl is provided on one of the fixing roller pair, which contacts with the non-image formation side of the recording medium. This makes it possible to perform the further stable feeding of the recording medium.
Furthermore, an image formation apparatus for attaining the above object according to the invention comprises a photosensitive member for forming thereon an electrostatic latent image by a signal from a host system, a development device for developing the electrostatic latent image on the photosensitive member and forming a toner image, a transfer device for transferring the developed toner image to a recording medium, and a fixing device for introducing the recording medium, to which the toner image has been transferred, between a pair of fixing rollers to fix the toner image on the recording medium. The pair of fixing rollers each have single silicone rubber layers coated around the outer peripheries of their core bars, which have a volume resistance value of 1013 xcexa9xc2x7cm or more.
In addition, the above-described image formation apparatus of the invention is constructed so that a surface of the fixing roller contacting with the unfixed toner image is charged with electricity of the same polarity as that of the toner particles supplied by the development device.